1. Field of the Invention
The present invention relates to a catalytic converter apparatus for purifying exhaust gas from an internal combustion engine (hereinafter, called an “engine”) for vehicles. More specifically, the present invention relates to a catalytic converter apparatus which purifies exhaust gas from an engine with a small amount of noble metal, and which is configured to decrease the exhaust gas pressure of the engine.
2. Description of the Related Art
With reference to FIG. 7, a description will be given below, of a process for purifying exhaust gas from an engine 10a of a commercial vehicle X.
First, the engine 100a discharges exhaust gas through cylinders within a cylinder block 101. Subsequently, the exhaust gas flows through an engine head block 102 and an exhaust manifold 103 where multiple exhaust pipes are joined together. Finally, the exhaust gas reaches a catalytic converter 109, and it is purified there.
The catalytic converter 109 holds catalysts 106b and 107b. Referring to FIGS. 8A and 8B, an exhaust gas catalyst 110 has a honeycomb structure 111 made of ceramic or metal which includes a catalyst support 113 and a wash coat layer 114. In addition, the wash coat layer 114 supports noble metal, such as platinum (Pt), palladium (Pd) or rhodium (Rh), that serves as a catalyst. Specifically, the exhaust gas catalyst 110 is formed by coating and baking the wash coat layer 114 on the honeycomb structure 111. The catalyst support 113 and the wash coat layer 114 define multiple cells 112. When exhaust gas passes through the cells 112, non methane organic gas (NMOG) and NOx, etc. contained in the exhaust gas is removed.
On the other hand, for the purpose of reducing environmental pollution, regulations against harmful substances such as NMOG and NOx are becoming stringent year by year. To give one example, Partial Zero Emission Vehicle (PZEV), which has been introduced by California, the U.S.A., is currently one of the strictest regulations for commercial vehicles. Only a handful of vehicles observe this regulation at present. Accordingly, catalytic converter apparatuses which exhibit an enhanced property of purifying exhaust gas have been in demand.
In order to comply with such strict regulations against harmful substances, various attempts to improve the purification property of catalytic converter apparatuses have been made. Some examples of those attempts are as follows:
1) to enlarge an area where exhaust gas comes into contact with the wash coat layer 114 (of FIG. 8B);
2) to grow the number of cells 112 in the honeycomb structure 111, that is, increase the density of the cells in each of the catalysts 106b and 107b; and
3) to raise the amount of the noble metal in the wash coat layer 114.
Thanks to the above attempts, the property of purifying exhaust gas by catalytic converter apparatuses has been enhanced. As a result, for example, in a test vehicle Y (see FIG. 3) equipped with an engine of 2.4 litter displacement, one catalyst has 900 cells/in2 and the other has 600 cells/in2. In addition, the vehicle contains noble metal of 12 g.
Moreover, Japanese Unexamined Patent Application Publication 2004-330025 discloses a catalyst provided with a wash coat layer having multiple voids. With these voids presented in the wash coat layer, exhaust gas is diffused widely and an area where the exhaust gas comes into contact with the catalyst is made large. This results in the improvement of the purification property.
However, when the density of the cells in a catalyst increases for the above reason, the exhaust gas resistance of the catalyst is prone to be high. This may cause the rise in the pressure of the exhaust gas, which lowers the output power of the engine. In addition, an amount of noble metal in a catalyst may increase, thus leading to high manufacturing costs for a vehicle.
Using of the catalyst disclosed by JP2004-330025 makes it possible to improve the purification property of catalytic converter apparatuses. However, this document fails to teach or suggest that the catalyst supports a small amount of noble metal, or that it is adapted to lower the exhaust gas pressure of the engine when being applied to an engine.
Taking the above description into account, the present invention has been conceived. An object of the present invention is to provide a catalytic converter apparatus for purifying exhaust gas from an engine, which achieves an improved purification property with a small amount of noble metal, and which is configured to decrease the resistance of exhaust gas.